The first verb ἦν is intransitive; it does not take an accusative object, so ὅ must be nominative.

The second verb ἀκηκόαμεν is transitive, but its subject must be first-person masculine plural. Since ὅ is neuter singular, it cannot be the subject and so is the expected direct object in the accusative. Ditto for ἑωράκαμεν and ἐθεασάμεθα.

Stephen Carlson wrote:Context helps of course, but here the syntax is a pretty clear guide:

The first verb ἦν is intransitive; it does not take an accusative object, so ὅ must be nominative.

The second verb ἀκηκόαμεν is transitive, but its subject must be first-person masculine plural. Since ὅ is neuter singular, it cannot be the subject and so is the expected direct object in the accusative. Ditto for ἑωράκαμεν and ἐθεασάμεθα.

Also, the article cannot be the subject of a verb except in the "rare" case that it functions as a personal pronoun (remnant of earlier Greek) such as in "ο δε ειπεν ..."